Most modern techniques for improving spectral resolution in NMR of solids include extremely rapid spinning of the sample at the “Magic Angle” (the zero of the second Legendre polynomial, 54.7°) with respect to B0. If the rotational rate is fast compared to chemical shift anisotropies and dipolar couplings (in units of Hz), the resolution is dramatically improved—often by two or three orders of magnitude. In many cases, it is important to be able to obtain the NMR data—often used to determine molecular structural information—on samples that are at very low or very high temperatures.
A technique sometimes capable of increasing signal to noise (S/N) ratio in MAS by one to three orders of magnitude in solid samples at low temperatures, known as Dynamic Nuclear Polarization (DNP), combines millimeter-wave (mmw) irradiation of the sample with NMR detection, where the mmw frequency is about 660 times the proton NMR resonant frequency. However the technique seldom works well above 120 K. Often, for each 10 K reduction in sample temperature between 120 K and 30 K, the S/N enhancement increases by a factor of two and the required mmw irradiation power decreases by a similar factor. Hence, there is strong motivation for improving MAS at temperatures below 120 K. In most cases, DNP works best at spinning rates between 4 kHz and 9 kHz, which is much less than desired for many other MAS techniques.
In U.S. Pat. No. 4,456,882, I disclose a high-speed NMR MAS ceramic sample spinner using radial bas bearings, a solid lubricated point bearing at the bottom, and impulse turbine drive at the top. In U.S. Pat. No. 4,511,841, Bartuska discloses a modified Beams-type Bernoulli out-flow bearing-drive for MAS; and in his later U.S. Pat. No. 4,940,942, he discloses a method of providing variable temperature (VT) operation for the sample using three gas stream—one for the sample region, one for the radial bearings at each end, and one for axial Bernoulli out-flow bearing and drive at the bottom. In U.S. Pat. No. 5,508,615, I disclose a method of suppressing whirl instability in the radial bearings at very high surface speeds in MAS and improving the stability of balanced axial hydrostatic bearings, similar to the one used in the HT-MAS probe disclosed in U.S. Pat. No. 5,202,633. In U.S. Pat. No. 7,151,374, I disclose a method of improving S/N in triple-tuned MAS probes by cooling the auxiliary RF tuning coils to about 100 K with a stream of cold N2 gas. In U.S. Pat. No. 7,170,292, we disclose a novel Bernoulli inflow axial bearing that is particularly advantageous for MAS when vacuum insulation is required between the rotor and the sample coils. This is advantageous in the CryoMAS probe we disclose in U.S. Pat. No. 7,282,919, or when the spinner needs to be hermetically sealed for operation inside an external high vacuum region, as disclosed in our improved CryoMAS probe in U.S. Pat. No. 7,915,893.
In all of the above cases except Bartuska's U.S. Pat. No. 4,940,942, the sample temperature is established predominately by the bearing gas temperature plus effects from frictional heating and RF heating, which is discussed in more detail by Doty et al in J. Magn. Reson. 182 (2006) pp 239-253. Bartuska correctly claims that using a separate cold gas stream for the sample VT with warm gas for the bearing and drive permits faster sample spinning at low temperatures, but the three-stream approach comes with its own set of problems: (1) the rotor must be much longer to reduce thermal gradients within the sample, which has prevented it from being used in narrow-bore (NB) magnets or even in wide bore (WB) magnets with sample eject; (2) access to the rf coils is considerably more complicated, and this has apparently prevented the advantageous use of multiple sample rf coils in low-temperature (LT) MAS probes with 3-stream operation. See, for example, the very impressive LT-MAS work described by Thurber and Tycko in J. Magn. Reson. 195 (2008) 179-186, in which they were able to achieve 6.7 kHz MAS at 25 K with a 4-mm rotor inside a probe of 88-mm OD in a 9.4-T magnet using a spinner similar to that of U.S. Pat. No. 4,940,942.
An estimated 85% of the NMR magnets sold between 2002 and 2012 have bores inside their room-temperature (RT) shims of less than 45 mm, and most of those have been 40 mm. Most (perhaps almost all) of the MAS probes for such have utilized two gas streams—one supplying bearing pressure and largely establishing sample temperature, and the other supplying pressure to the drive turbines. Most of these probes have been specified by their manufacturer (such as Bruker, Agilent, or JEOL) as being able to spin at temperatures down to around 200 K. A substantial number of probes by another manufacturer (Doty Scientific) have been specified as being able to spin at temperatures down to 110 K to 160 K using N2, but it has always been very difficult to obtain stable spinning at acceptable speeds at temperatures below 130 K for extended periods of time with known N2 gas-cooling technology—and sometimes difficult even at 180 K.
Prior Art MAS Cold Gas Supplies. For the past three decades, most MAS cold-gas supply systems have been similar in general respects to that shown in FIG. 1. Pressurized RT N2 gas is pre-cooled by boil-off gas flowing up the neck of the cryostat, and then cooled by a cooling coil immersed in liquid nitrogen (LN2) in the LN2 cryostat (often 50-liter capacity).
The biggest limitation with prior art LT-MAS using two N2 gas streams comes not from the spinner or probe design, but from the cold gas supply to the probe. It has not been practical to reduce heat leaks in the flow between the spinner gas cooling coil in the LN2 cryostat and the spinner assembly to less than about 15 W using standard vacuum-insulated transfer lines and couplings. Typical losses (for either the bearing or the drive stream) are ˜2.5 W in the connections at each end of the vacuum-insulated transfer line, ˜3 W per additional connection (in the base of the probe and at the top of the top of the LN2 cryostat), ˜3 W in the flexible vacuum insulated transfer line, and ˜1 W miscellaneous, totaling ˜15 W for each stream.
However, the bearing gas flow rate needed for a typical 4-mm rotor, for example, is only about 0.25 g/s at ˜110 K for ˜5 kHz spinning. At that flow rate and heat leak, the temperature of the bearing gas—if no liquid is present—increases by 56 K between the cooling coil in the liquid nitrogen cryostat and the spinner. If nitrogen gas leaves the cooling coil just above its boiling point at 220 kPa (which is 84 K), it arrives at the spinner at 140 K. The problem is worse with smaller spinners, as the gas flow rate is lower.
The temperature rise during the transfer can be reduced by deliberately adding a vent hole near the spinner to increase the flow rate, but even with a leak three times the normal bearing flow, the minimum bearing gas temperature entering the spinner will be ˜105 K, and the large additional gas flow is not desired, particularly for long runs.
To achieve bearing-gas temperatures below ˜140 K at low flow rates it is necessary to have a liquid fraction in the gas leaving the cooling coil and to use its heat of vaporization to balance the heat leaks. At a liquid fraction of 30% for 0.25 g/s N2, the heat of vaporization of the liquid is 14 W—about what is needed to balance the heat leaks through the vacuum-insulated transfer line, couplings, and dewared transfer line inside the probe. So if it were possible to achieve a 30% liquid fraction (by mass, not volume) of N2 leaving the cooling coil at 84 K and 220 kPa, the 0.25 g/s flow should arrive at the spinner as 100% gas at 84 K. Unfortunately, this is an impossible control problem by prior methods, as explained in the following discussion.
Oscillating Flow with Conventional Two-phase Cooling. If liquid droplets reach an orifice in the spinner assembly (or a leak along the way), the mass flow increases dramatically. (The viscosity of liquid nitrogen is an order of magnitude greater than that of the gas, but the density of the liquid may be two orders of magnitude greater at typical conditions.) At the higher flow rate, the pressure in the transfer line from the gas cooling coil plummets, causing the temperature to drop and the flow rate through the cooling coil in the LN2 cryostat to jump. Since there is a significant liquid fraction in the fluid in the transfer line, its temperature stays pegged at the boiling point of N2 at the pressure in the line until the liquid fraction is gone.
If a vent hole has been added near the spinner with leak rate similar to the bearing flow, the total flow needed (for the typical 4-mm spinner) is ˜0.5 g/s, and the ideal starting liquid fraction of N2 should be ˜15%. If the mixture in the transfer line is 15% liquid at 0.5 g/s, it runs through the transfer line in about half a minute for the typical case. However, when liquid is at the bearing orifices, the flow rate is much higher than if all gas, so the mixture coming from the cooling coil at the lower transfer-line pressure would be at a slightly lower temperature but of decreased liquid fraction (because the cooling capacity of the cooling coil is less at a temperature closer to the reservoir temperature of 77 K in the 50-L cryostat).
So the liquid fraction in the bearing gas leaving the cooling coil is now insufficient to prevent a substantial rise in its temperature from the heat leaks during the transfer. By the time the new mixture gets to the stator, it is above the liquid boiling point, so the mass flow drops because it contains no liquid. The bearing pressure then jumps, the liquid in transit stops vaporizing, and the temperature jumps. The liquid fraction produced in the cooling coil also jumps (because the flow rate has dropped), the transfer line begins filling again with a mixture of higher liquid fraction, and the cycle then repeats when the high-liquid-fraction fluid gets to the stator.
Some positive feedback (the basic cause of instability) is also present in the system even without liquid flow through the stator (or leaks) since the gas viscosity decreases and its density increases as the temperature decreases, but this control problem is manageable until liquid droplets get to leaks or bearing holes.
Barnes et al in J Magn. Reson 198 (2009) pp. 261-270, describe a method of addressing what they see as the root of the instability problem—condensation in the cooling coil. In essence, they maintain the LN2 in the cryostat bathing the cooling coil at a pressure higher than the maximum pressure inside the cooling coil. In this way, the temperature of the LN2 is easily maintained above that which leads to condensation inside the cooling coil. While they report some success in improving stability, it should be noted that their magnet RT bore is 130 mm, the probe is entirely enclosed in a dewar of 127 mm OD, and extraordinary measures are taken to minimize heat leaks into the cold-gas transfer lines. Apparently, they have been able to reduce heat leaks sufficiently to avoid the need for a significant liquid fraction in the cold gas, but such measures are not possible in most laboratories.